This invention relates to apparatus for controlling the load-bearing capacity of the hydraulic props of a mine roof support unit, particularly a mine roof support unit having hydraulic props whose inclination to the vertical increases as the props are extended. Throughout this specification the term "vertical", when used with reference to the inclincation of the hydraulic props of a mine roof support unit, should be taken to mean the direction perpendicular to the floor of the working on which the mine roof support unit stands.
A known type of mine roof support unit for use in a longwall working has a roof cap supported on a floor sill by a pair of hydraulic props. The hydraulic props are articulatedly attached to both the floor sill and the roof cap, and are inclined so that, in use, their upper ends are nearer the longwall face than their lower ends. This type of roof support unit usually has a goaf shield pivotally connected between the goaf-side ends of the roof cap and the floor sill. The goaf shield is preferably connected to the floor sill by a pair of links constituting a lemniscate linkage. As the hydraulic props are extended and retracted, they each pivot in a vertical plane perpendicular to the face. Thus, as the props are extended, they become increasingly inclined relative to the vertical.
Roof support units of this type are designed to be used in longwall workings of differing heights. Obviously, the units are required to support the roof of a longwall working no matter how high its roof is. Consequently, the hydraulic props of roof support units of this type need to have a sufficient load-bearing capacity that the vertical component thereof (that is to say the component available for roof support) is sufficiently large to support the roof, even when the props are fully extended and at their maximum angle of inclination to the vertical. Unfortunately, by having props of sufficient load-bearing capacity to support the roof under maximum inclination conditions, the vertical component of the load-bearing capacity when the props are fully extended (that is to say when their inclination to the vertical is a minimum) is far too high. In particular, the roof cap may be pressed against the roof with sufficient force to damage the roof, and the floor sill may be forced down into the floor of the working. Moreover, the roof support unit has to be of excessively heavy construction to take up the large upward forces which occur at maximum prop extension.
The aim of the invention is to provide apparatus for controlling the load-bearing capacity of the inclined hydraulic props of a mine roof support unit so that the above-mentioned disadvantages are obviated.